1. Field of the Invention
The present invention relates to multi-track magnetic signal reproducing apparatuses using a multi-track magneto resistance effect (hereinafter referred to as MR) head.
2. Description of the Background Art
When an MR head is brought into contact with a running recorded magnetic tape, the resistivity of an MR element built in the MR head changes in response to a magnetic field from the magnetic tape. The MR head reproduces a signal recorded on the magnetic tape taking advantage of this change.
The operation principles of the MR head will be described in conjunction with FIGS. 8 and 9. In FIG. 8, a constant current power supply 1 is connected to an MR element (indicated by MR in the figures) 2. When a bias magnetic field is applied to MR element 2, resistance R slightly changes in proportion to --H, where H represents the intensity of the magnetic field from the magnetic tape and R represents the resistance of MR element 2. When a constant current I is passed across MR element 2 with constant current power supply 1, voltage V at a terminal d is V=R*I. Resistance R slightly changes in proportion to the intensity H of the magnetic field, and voltage V slightly changes accordingly. Therefore, the magnetic signal recorded on the magnetic tape can be reproduced using the change .DELTA.V in voltage at terminal d. When only a bias magnetic field is applied to MR element 2, the resistance of MR element 2 is R.sub.0 and voltage at terminal d is V.sub.0 (=R.sub.0 *I) for the intensity H of the magnetic field from the magnetic tape being 0, an output voltage V.sub.out by a reproducing signal is represented as .DELTA.V (=V-V.sub.0) produced by excluding voltage V.sub.0 (offset voltage) from detected voltage V at terminal d. When voltage V at terminal d is amplified, output voltage V.sub.out by a very small reproducing signal included therein is amplified, and offset voltage V.sub.0 is amplified as well. Accordingly, reproducing processing with high accuracy cannot be made because of offset voltage V.sub.0.
FIG. 9 illustrates one example of a blocking arrangement for amplifying only output voltage V.sub.out by the reproducing signal exclusive of offset voltage V.sub.0 described in conjunction with FIG. 8. In FIG. 9, in addition to constant current power supply 1 and MR element 2 shown in FIG. 8, a capacitor 15 and an amplifier 16 are further included. Amplifier 16 amplifies output voltage V.sub.out provided based on the amount of gain of as much as 60 dB, for example, for output to a terminal e. Voltage V.sub.out is in the range from several ten .mu.V.sub.P--P to several hundred .mu.V.sub.P--P, and therefore the voltage reaches to a range from several ten mV.sub.P--P to several hundred mV.sub.P--P after amplification by amplifier 16. According to 8/10 modulation system by which a bit rate per one channel (one recording track on a tape) is 96 k bit/sec and 8-bit data is converted into data of 10 bits to be recorded on a tape, a reproducing frequency having the shortest recording wavelength is 48 kHz. Generally, a reproducing frequency at least 1/100 of 48 kHz, in other words at least 480 Hz will be necessary. Therefore, when the value of resistance R of MR element 2 is set to 100.OMEGA., and a cut off frequency to 480 Hz, capacitor 15 will have a capacitance of about 3.3 .mu.F, which is relatively large for a capacitor.
The multi-track MR head system has a plurality of such MR heads arranged widthwise of a magnetic tape, with the MR heads provided individually corresponding to a plurality of tracks arranged widthwise of the magnetic tape, and a magnetic signal on each track is simultaneously reproduced by a corresponding MR head. For example, for N tracks provided on a magnetic tape, the multi-track MR head system includes at least N MR heads.
FIG. 10 is a block diagram showing a multi-track magnetic signal reproducing apparatus using a multi-track MR head in Conventional Example 1. The apparatus includes constant current power supplies 1-1 to 1-N, MR elements 2-1 to 2-N, an A/D (Analog/Digital) converter 5, a waveform equalizing circuit 9, a data detection circuit 10, an RAM (Random Access Memory) 11, a signal processing circuit 12, capacitors 15-1 to 15-N, amplifiers 16-1 to 16-N, a counter 17 and an N-input multiplexer 18. A magnetic tape has a plurality of tracks, and a magnetic signal recorded on each track is reproduced by the output voltage of an MR element provided corresponding to each track. The output voltage of MR element 2-j provided corresponding to the j-th track (for j=1, 2, 3, . . . , N), for example, has its DC (Direct Current) component cut by capacitor 15-j, then amplified to a prescribed level by amplifier 16-j and then provided to multiplexer 18. Counter 17 controls multiplexer 18 to sequentially select one of the N inputs and conducts a selected one to the output side, while counting from 1 to N. Accordingly, N outputs from all these amplifiers are subjected to parallel-serial conversion and provided to A/D converter 5. A/D converter 5, waveform equalizing circuit 9 and data detection circuit 10 process signals for N tracks in a time-dividing manner. More specifically, A/D converter 5 quantizes and produces a discrete reproducing signal for each track, and provides a resultant digital signal to waveform equalizing circuit 9, waveform equalizing circuit 9 in turn removes the input intersymbol interference of digital signal with code for application to data detection circuit 10, and data detection circuit 10 converts the digital signal into a binary value of 0 and 1 by zero cross determination or the like and has the resultant data written and recorded in RAM 11. Signal processing circuit 12 reads out digital information recorded in RAM 11, performs processings such as modulation and error correction, and then externally outputs the information from terminal a. Conventional Example 1 requires N capacitors for removing DC component.
Note that information recorded on a magnetic tape includes an audio sound, a still picture, a motion picture and sentences.
In FIG. 10, portion H surrounded by the dotted line is a multi-track MR head. For the terminals of the multi-track MR head, N terminals connected to current power supplies and capacitors and each for outputting a magnetic signal detected by each MR element, and a ground terminal are provided. Accordingly, the number of terminals of the multi-track MR head should be N+1 in total.
When the current power supplies and the amplifiers are manufactured into an IC (Integrated Circuit), for terminals for the IC, the input terminal of each amplifier, the output terminal of each amplifier, the current supply terminal of each current power supply, a power supply terminal, a ground terminal and the like will be necessary. For the number of terminals .alpha. for such as the power supply terminal and the ground terminal, the number of terminals for the IC will be 3N+.alpha. in total.
The above-described approach passes constant current across the MR elements, but an approach of passing pulse current is known (IBM Technical Disclosure Bulletin Vol. 19 No. 8 January 1977, pp. 3222-3223).
According to this approach, output voltage obtained at the time of passing pulse current is A/D converted to provide a reproducing signal. Also in this approach, output voltage by a reproducing signal equivalent to the case of passing constant current is obtained, and power consumption can be reduced, because current is passed only when the pulse current is supplied. Passing pulse current several times as large at a level as the constant current provides output voltage at a level several times as large.
In Conventional Example 1 shown in FIG. 10, constant current flowing across the MR elements in total is N times as large, because the number of tracks is N times as large, which increases power consumption by the amount. A multi-track type magnetic signal reproducing apparatus by which the total amount of current flowing across MR elements does not increase even the number of tracks is N times as large when the above-described approach of passing pulse current across each MR element is employed (see Japanese Patent Laying-Open No. 61-148610). This approach of passing pulse current across each MR element is referred to as Conventional Example 2, and FIG. 11 illustrates a block arrangement of Conventional Example 2.
In FIG. 11, the apparatus includes MR elements 2-1 to 2-N, a switch 3, an A/D converter 5, a waveform equalizing circuit 9, a data detection circuit 10, an RAM 11, a signal processing circuit 12, an MR driver circuit 13, a pulse generator 14, amplifiers 16-1 to 16-N, a counter 17 sequentially counting from 1 to N, and an N-input multiplexer 18. A pulse signal generated by pulse generator 14 is supplied to MR driver circuit 13, and functions to cause MR driver circuit 13 to output pulse current. The output pulse current is sequentially switched by switch 3, and sequentially applied to MR elements 2-1 to 2-N, and voltage output from each MR element in response is amplified by a corresponding one of amplifiers 16-1 to 16-N, and is applied to N-input multiplexer 18 in parallel. Counter 17 inputs the pulse signal of pulse generator 14, and in response counts, and controls multiplexer 18 to sequentially select one of N inputs for output to A/D converter 5 in synchronization with the switching operation of switch 3. A/D converter 5 inputs the output signal of each MR element from multiplexer 18, and A/D converts the input signal after the signal transits to a steady state, and applies the converted signal to waveform equalizing circuit 9. The signal processing after waveform equalizing circuit 9 is the same as the case in Conventional Example 1 illustrated in FIG. 10, and therefore a description thereof is omitted.
Herein, for the terminals of the multi-track MR heads in the portion H surrounded by the dotted line in FIG. 11, N+1 in total including N terminals connected to switch 3 and amplifiers 16-1 to 16-N for outputting voltages by magnetic signals detected by the MR elements and a ground terminal will be necessary. When MR driver circuit 13, pulse generator 14, switch 3 and amplifiers 16-1 to 16-N are manufactured into an IC, for the number of terminals for the IC, a terminal which functions for supplying current to each MR element via switch 3 and as an input for each amplifier, the output terminal of each amplifier, a power supply terminal, a ground terminal and the like will be necessary. For the number terminals such as the power supply terminal and the ground terminal being .alpha., the number of terminals for the IC in this case will be 2N+.alpha. in total.
In Conventional example 2 illustrated in FIG. 11, when each amplifier amplifies voltage V.sub.out output by a corresponding MR element, offset voltage V.sub.0 included therein is simultaneously amplified. Offset voltage V.sub.0 is for example as large as several hundred mV, and therefore the gain of the amplifier cannot be increased. Accordingly, removal of offset voltage V.sub.0 in a preceding stage to the amplifier may be possible, but since pulse current rather than constant current is passed across each MR element, provision of a capacitor for cutting DC component in a preceding stage to the amplifier as in the case of Conventional Example 1 cannot remove offset voltage V.sub.0. Therefore, implementation of signal processing after the amplifier is very difficult.
Also in Conventional Example 2 in FIG. 11, since each amplifier must amplify an output voltage V.sub.out having pulse wave, as many as N amplifiers responding at a high speed in time with a short pulse width must be used, which pushes up the cost.
Furthermore, in Conventional Examples 1 and 2, as many as N+1 terminals are necessary for the multi-track MR head surrounded by the dotted line H, particularly in Conventional Example 1, when N current power supplies and N amplifiers are manufactured into an IC, N capacitors must inevitably be attached externally, which requires at least 3N or 2N terminals for the IC, resulting in difficulty in packaging of the IC. Accordingly, the connection between the multi-track MR head and the amplifiers is complicated, and therefore the multi-track magnetic signal reproducing apparatus cannot be manufactured inexpensively.